The openings in the plastic sheet allow penetration of the plastic foam into the fibrous board to effect intermittent-site bonding of the board to the foam layer. If the openings are too numerous or too large, too much foam penetrates the fibrous glass board, raising the materials cost unnecessarily. If the openings are too few, too small, or improperly sited, too little foam penetrates the fibrous glass board, lowering the bonding efficiency and strength of the composite.
A need exists for an inspection apparatus to determine the deployment of the openings in a sheet, preferably a plastic sheet, before the sheet contacts the liquid foam, and a means to translate the deployment data to useable inspection, and quality and process control form.
Sensing of openings in sheets, cards, and tapes is well-known. Examples include player piano rolls, business machine punched cards, and teletype punched tape. The perforated sheet used in composite panels of roof insulation presents particular problems in sensing of openings in the sheet. The openings are about 0.040 inch in diameter and are spaced on about 0.5 inch centers. Accordingly, about one-half-of one percent of the surface of the sheet is comprised of openings. Optical and pneumatic sensors are incapable of sufficient resolution to sense deployment differences for such small holes, and are adversely affected by even slight misalignments of the sheet. Mechanical contacts such as fingers or probes are undesirably complicated for a sheet with such small holes at any significant machine speed. Additionally, sheet durability and misalignment sensitivity militate against use of mechanical contacts. The need demands a sensing apparatus comprising a large sensing field that is not adversely affected by sheet alignment, opening deployment, or high machine speed.